1. Field of the Invention
The present invention relates to bandgap voltage reference generators, and in particular, to startup circuits used to ensure that the host bandgap voltage reference generator circuit enters and remains in a valid and stable operating state following the initial application of DC power.
2. Description of the Related Art
As is well known, bandgap voltage reference generator circuits are used to provide stable reference voltages over wide variations in operating temperatures. One problem associated with these types of circuits, however, involves circuit startup characteristics. For example, a fast rise time in the power supply voltage will generally result in reliable circuit startup. However, startup may not occur if the power supply voltage increases slowly and the operating temperature is low due to the fact that low current beta characteristics of the bipolar circuit elements (due to the low temperature) are often too low to provide the leakage currents needed to produce current regeneration necessary to initiate proper circuit operation. Furthermore, even if the circuit initially starts up properly, if the power supply voltage drops low enough to shut down the circuit, the circuit may not turn back on properly once the power supply voltage has been restored to its proper value. This problem of circuit startup due to slow or marginal power supply voltages has become even more significant with increasing uses of circuits operating at very low power supply voltages.
In accordance with the presently claimed invention, a startup circuit for a bandgap voltage reference generator circuit monitors an internal reference voltage of a bandgap voltage reference generator circuit and initiates current flow for the bandgap circuit diodes following circuit startup, e.g., initial application of DC power. In one embodiment, the monitored bandgap circuit reference voltage is replicated and used to assert and then de-assert a control signal used for initiating the diode current flow.
In accordance with one embodiment of the presently claimed invention, a startup circuit for a bandgap voltage reference generator circuit includes a reference terminal, a control terminal and voltage monitoring circuitry. The reference terminal conveys a reference voltage including first and second voltage magnitudes related to a current flow in a bandgap voltage reference generator circuit. The control terminal conveys a current control signal for initiating the current flow. The voltage monitoring circuitry, coupled to the reference and control terminals, provides the current control signal in response to reception of the reference voltage. The current control signal includes an asserted state corresponding to the first reference voltage magnitude and a de-asserted state corresponding to the second reference voltage magnitude, and the current flow initiation occurs in response to the asserted state.
In accordance with another embodiment of the presently claimed invention, a startup circuit for a bandgap voltage reference generator circuit includes voltage replication circuitry and signal driving circuitry. The voltage replication circuitry provides a replica voltage responsive and corresponding to reception of a reference voltage including first and second reference voltage magnitudes related to a current flow in a bandgap voltage reference generator circuit. The signal driving circuitry, coupled to the voltage replication circuitry, provides a current control signal in response to reception of the replica voltage. The current control signal includes an asserted state corresponding to the first reference voltage magnitude and a de-asserted state corresponding to the second reference voltage magnitude, and the current flow initiation occurs in response to the asserted state.